A U.S. State Department spokesperson said Russia had notified it that a U.S. citizen had been detained and it expected Moscow to provide consular access to see him.
"We have requested this access and expect Russian authorities to provide it," the spokesperson said, without providing details of the identity of the American or the reasons behind his detention.
The English-language service of TASS news agency named him as Paul Whelan but Reuters was unable to independently confirm the exact spelling of his name. The State Department did not identify him.
Russia's relations with the United States plummeted when Moscow annexed the Crimean peninsula from Ukraine in 2014, and Washington and Western allies have imposed a broad range of sanctions on Russian officials, companies and banks.
Earlier this month Russian national Maria Butina pleaded guilty in a U.S. court to a conspiracy charge in a deal with prosecutors, and admitted to working with a top Russian official to infiltrate American conservative activist groups and politicians as an agent for Moscow.